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SubjectRe: [PATCH v4 00/10] steal tasks to improve CPU utilization
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Hi,

On 10/12/2018 16:29, Steven Sistare wrote:
[...]
>> I have run some hackbench tests on my hikey arm64 octo cores with your
>> patchset. My original intent was to send a tested-by but I have some
>> performances regressions.
>> This hikey is the smp one and not the asymetric hikey960 that Valentin
>> used for his tests
>> The sched domain topology is
>> domain-0: span=0-3 level=MC and domain-0: span=4-7 level=MC
>> domain-1: span=0-7 level=DIE
>>
>> I have run 12 times hackbench -g $j -P -l 2000 with j equals to 1 2 3 4 8
>>
>> grps time
>> 1 1.396
>> 2 2.699
>> 3 3.617
>> 4 4.498
>> 8 7.721
>>
>> Then after disabling STEAL in sched_feature with echo NO_STEAL >
>> /sys/kernel/debug/sched_features , the results become:
>> grps time
>> 1 1.217
>> 2 1.973
>> 3 2.855
>> 4 3.932
>> 8 7.674
>>
>> I haven't looked in details about some possible reasons of such
>> difference yet and haven't collected the stats that you added with
>> patch 10.
>> Have you got a script to collect and post process them ?
>>

I used the script that Steve sent just before LPC [1] - I probably should
have given that a try sooner...


Running the base "hackseries" on my H960 gave me this:

--- base -- --- new ---
groups time %stdev time %stdev %speedup
1 1.021 9.1 1.214 9.1 -15.9
2 1.066 4.3 1.232 7.1 -13.5
3 1.140 9.3 1.247 3.0 -8.6
4 1.207 5.4 1.246 6.2 -3.2


Now that board struggles with thermal, so I swapped the order of testing
(STEAL enabled first, then NO_STEAL) out of curiosity:

--- base -- --- new ---
groups time %stdev time %stdev %speedup
1 0.986 8.6 1.218 4.9 -19.1
2 1.096 5.5 1.290 6.2 -15.1
3 1.124 5.5 1.237 8.1 -9.2
4 1.181 8.7 1.238 5.9 -4.7


And actually running the same test twice with NO_STEAL gives me:

--- base -- --- new ---
groups time %stdev time %stdev %speedup
1 1.005 8.3 1.225 5.5 -18.0
2 1.126 6.4 1.220 7.1 -7.8
3 1.199 5.8 1.264 3.2 -5.2
4 1.167 4.6 1.314 8.5 -11.2


We might need some other benchmark to test this. Or a much bigger fan...

[1]: https://lore.kernel.org/lkml/0eaa3ee9-64d6-4739-eec9-e28befa0e97f@oracle.com/

>> Regards,
>> Vincent
>
> Thanks Vincent. What is the value of /proc/sys/kernel/sched_wakeup_granularity_ns?
> Try 15000000. Your 8-core system is heavily overloaded with 40 * groups tasks,
> and I suspect preemptions are killing performance.
>

While hackbench is not super representative of "real life", I wonder if we
shouldn't do something about the default if using stealing suggests it
(wild speculation here).

> I have a python script to post-process schedstat files, but it does many things
> and is large and I am not ready to share it. I can write a short bash script if
> that would help.
>
> - Steve
>

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